1. Technical Field
This invention relates in general to devices and methods for in-vivo measurement of blood hematocrit and, more specifically, to devices and methods for such measurement that use impedance and pressure plethysmography.
2. State of the Art
The "hematocrit" of blood, which is defined as the percentage of whole blood volume occupied by erythrocytes (i.e., red blood cells), is an important measure of patient well-being in cases of trauma, blood loss by disease, iron depletion in pregnancy, dietary iron deficiency, and a number of more specific medical conditions.
Hematocrit has traditionally been measured by centrifuging a column of blood, which has been extracted from the patient, in a glass tube, until the erythrocytes are compacted by centrifugal force to one end of the tube. The hematocrit is determined by measuring the length of the tube containing dark red material and dividing by the total length of the liquid column in the tube. These length observations are usually made visually, but are also made, in some cases, by automated optical means of various designs. Besides centrifugal hematocrit determinations, hematocrit is also derived and reported by various automated blood analyzers which count erythrocytes optically in unpacked blood. This erythrocyte count correlates with packed cell hematocrit and the derived hematocrit is reported.
It is noted that the methods of obtaining hematocrit described above are invasive, that is, they require that blood be removed from the patient in order to determine the hematocrit. A non-invasive method would be desirable because it would subject the patient to less pain and inconvenience and would preserve the patient's blood for its normal functions.
It has long been recognized by biomedical researchers that the electrical impedance of blood varies with hematocrit and that, as a result of this relationship, it should be possible to derive hematocrit from the measurement of blood impedance. This has been successfully done on blood which has been extracted from the patient and placed in an impedance measuring cell of controlled dimensions, where the blood is stationary, maintained at a known temperature, and agitated to maintain uniform cell distribution. Examples of such successful measurements are given by Okada and Schwan in "An Electrical Method to Determine Hematocrits," IRE Transactions in Medical Electronics, ME-7:188-192 (1960) and by deVries et al. in "Implications of the Dielectrical Behavior of Human Blood for Continuous Online Measurement of Hematocrit," Medical & Biological Engineering and Computing, pp.445-448 (1993). Like the centrifugal methods, these methods are invasive, however, and thus do not satisfy the need for a non-invasive hematocrit measurement. The impedance methods have, however, provided the inspiration for some ingenious inventions to measure hematocrit in-vivo and non-invasively.
The first in-vivo impedance measurement of hematocrit known to the inventors was reported by Yamakoshi et al. in "Noninvasive Measurement of Hematocrit by Electrical Admittance Plethysmography Technique, " IEEE Transactions, BMB-27, 3:156-161(1980). This measurement was made by immersing the finger of the test subject in a saline solution contained in a chamber fitted with impedance measuring electrodes. The electrolyte concentration of the saline solution was then varied by mixing in either water or more concentrated saline until the pulsatile variations of impedance caused by the increased volume of blood on each pulse were minimized. When this minimization of pulses occurred, the saline solution had the same resistivity as the blood in the pulsing arteries and this resistivity could be correlated against the known, previously determined relationship between resistivity and hematocrit.
U.S. Pat. No. 5,526,808 by Kaminsky, assigned to Microcor, Inc., the assignee of the present invention, describes another impedance method for measuring hematocrit non-invasively and in-vivo. This method draws upon the observation that hematocrit determines the frequency vs. impedance profile of blood. In addition, the method of the '808 patent uses the pulsatile change of impedance in a finger or other limb of the body that occurs when each heartbeat pushes new blood into the organ where the measurement is made to separate the non-blood tissue impedance from the blood impedance.
The mathematical model upon which this method is based relies upon the assumption that the admittance (i.e., the reciprocal of impedance) change that occurs when blood pulses into the finger or other limb where the measurement is being made is due to the increased volume of blood providing a new current path in parallel with the old current path present before the pulse occurs. Thus, the difference in admittance between baseline, when no new blood is in the limb, and during the pulse, when new arterial blood has entered the limb, is due to the new blood, and the numerical value of this admittance difference is proportional to the volume of the new blood times the admittance of the new blood.
As shown in devries et al., the admittance vs. frequency characteristics of blood have a characteristic shape that depends upon hematocrit. Comparing the shapes of either the magnitude or the phase versus the frequency of the admittance, derived for the pulsed blood, against known characteristic hematocrit-dependent shapes gives a measure of hematocrit. The known characteristic shapes can be derived from a database obtained from patients having hematocrits independently measured by the centrifugal method previously described.
U.S. Pat. No. 5,642,734 to Ruben et al., assigned to Microcor, Inc., assignee of the present invention, describes some additional methods to obtain in-vivo hematocrit results. First, the '734 patent describes using pressurized cuffs, in various ways, to change the amount of blood in the organ (e.g., the finger) under measurement. Second, the '734 patent describes a unique electronic system for driving electrodes attached to the body part under measurement and for deriving phase as well as amplitude information from impedance measurements of the body part. Third, the '734 patent teaches the use of a neural network computer algorithm to relate measured impedance and other data to hematocrit based upon matching a database obtained from a number of prior measurements of patients with separately-determined hematocrits.
In the field of blood oxygen saturation measurement, as opposed to the field of blood hematocrit measurement that has been under discussion thus far, U.S. Pat. No. 5,111,817 to Clark et al. observes that the accurate measurement of blood oxygen saturation levels in arteries (S.sub.a O.sub.2) in a body part under measurement, such as a finger, is typically hindered by different blood oxygen saturation levels in capillaries (S.sub.c O.sub.2) in the body part. Clark et al. teaches a method for correcting measurements of S.sub.a O.sub.2 for the effects of S.sub.c O.sub.2. In this method, a pressure cuff applies a pressure to the body part under measurement that is equal to the mean arterial blood pressure in the body part. As a result, measurements from the body part are dominated by the effects of the actual S.sub.a O.sub.2 in the body part, so that the measured S.sub.a O.sub.2 is closer to the actual S.sub.a O.sub.2.